Got my bike mounts in the other day and got one installed. Would probably be better to screw this on a proper seat post clamp but I like the military look of the mount and O ring. Still if the O ring pops you'd probably drop this light and never know it. At 10 bucks a light it's not the end of the world but still.
And I suspect this O ring was intended for a smaller sized seat post than the thicker Dahon seatposts. It fits but it's a stretch.
Scratches on the seat post are unrelated to this. They came from a clamp the original owner had on the post.